Device for electrically heating a semiconductor rod which is simultaneously growing due to a depositing process from the gas phase

ABSTRACT

A device for electrically heating a semiconductor rod which is simultaneously growing due to a depositing process from the gas phase employs a heating current source for supplying an alternating voltage and an electronic switch which is operated by a time wise variable auxiliary voltage for controlling the application of heating current supplied by the heating current source to the semiconductor rod. A second circuit is coupled to the aforementioned circuit to rectify the applied voltage and pass the rectification product to a four-pole smoothing filter circuit. The differential voltage at the output of the four-pole circuit, with respect to a reference voltage, serves to control a generator for generating the auxiliary voltage in such a way that the auxiliary voltage is applied earlier in response to larger values of difference voltage.

United States Patent [191 Stut PHASE [75] Inventor:

Hans Stut, Groebenzell, Germany Siemens Aktiengesellschaft, Berlin and Munich, Germany Filed: July 5, 1972 Appl. No.: 269,154

Assignee:

[30] Foreign Application Priority Data July 7, 1971 Germany .1 2133863 [56] References Cited UNITED STATES PATENTS 11/1931 Houck 333/79 9/1935 Rose. 5/1957 Davis 23/301 SP 5/1967 Pinckaers 323/24 3/1968 Pinckuers 323/24 Aug. 27, 1974 3,447,902 6/1969 Benedict et a1, 23/301 SP 3,532,900 10/1970 Rhyne 323/24 X 3,538,427 11/1970 Oltendorf 323/24 3,646,423 6/ 1970 Tatematsu et al. 3,700,412 10/1972 Higashi et al.... 3,723,854 3/1973 Kita 323/4 Primary ExaminerWilliam H. Beha, Jr. Attorney, Agent, or Firml-lill, Gross, Simpson, Van Santen, Steadman, Chiara & Simpson [57] ABSTRACT A device for electrically heating a semiconductor rod which is simultaneously growing due to a depositing process from the gas phase employs a heating current source for supplying an alternating voltage and an electronic switch which is operated by a time wise variable auxiliary voltage for controlling the application of heating current supplied by the heating current source'to the semiconductor rod. A second circuit is coupled to the aforementioned circuit to rectify the applied voltage and pass the rectification product to a four-pole smoothing filter circuit. The differential voltage at the output of the four-pole circuit, with respect to a reference voltage, serves to control a generator for generating the auxiliary voltage in such a way that the auxiliary voltage is applied earlier in response to larger values of difference voltage.

13 Claims, 6 Drawing igures i ll DEVICE FOR ELECTRICALLY HEATING A SEMICONDUCTOR'ROD WHICH IS SIMULTANEOUSLY GROWING DUE TO A DEPOSITING PROCESS FROM THE GAS PHASE BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a device for electrically heating a semiconductor rod which is simultaneously growing during heating due to a depositing process, and is particularly concerned. with such a device which comprises a heating source which supplies a periodically alternating voltage, an electronic switch controlled by a time variable alternating voltage for controlling the application of heating current, and means for producing the alternating voltage for controlling the electronic switch.

2. Description of the Prior Art Inasmuch as a semiconductor rod, in particular a silicon rod, has a decreasing temperature-resistance characteristic while growing due to a depositing process from a gas phase, and therefore also a decreasing voltage characteristic, and inasmuch as the entire impedance of the rod successively decreases due to its growth, special considerations must be given to the planning of a device for electrically heating such a semiconductor rod while it is growing.

From a thermal point of view, therod is in a stationary heat balance during operation wherein the rod releases as much heat to its surroundings as is developed in heating the rod. This stationary balance changes with increasing rod diameter in such a way that the performance required for achieving stationary heat balance must be gradually increased. Due to the decresing voltage characteristic of a silicon rod, provisions have to be taken into account in the electrical heating process that the heat developed in the rod does not exceed the heat dissipation in the rod. Generally, this is avoided by introducing stabilizing elements in the heating circuit. For this purpose, the stabilizing element can be present in the heating current source itself, which is the case in the so called load independent current sources. As described above, a constant temperature can be achieved by balancing power development and cooling in such a way that the desired rod temperature will occur. Since in the case of a constantly maintained 0 the heat dissipation to the surroundings of the rod is only determined by the surface of the silicon rod, the power supply must be adapted to the changing diameter d of the rod, which is constantly growing. Since, however, the deposition takes place at a comparatively slow rate whereby the rod diameter increases slowly, it is desirable to provide an adjustment of rod temperature whereby it corresponds to rod diameter. This means that the power supply must be adjusted to a power output level which experiences a gradual increase during the course of the growing process.

SUMMARY OF THE INVENTION If a load independent A.C. voltage source is provided as the heating energy source, such as is usually supplied by the electrical service mains, acontrol can be traced to control of the effective current.

The present invention deals with the foregoing type of arrangement. A concept of the present invention is to control an electronic switch in the heating circuit in such a way that an adequate power supply is guaranteed. The control is therefore effective on the heating circuit, whereby the adjusting value is constituted by the actual value of the heating current itself.

However, inasmuch as the silicon rod is an unstable structure, electrically as well as thermally, due to its decreasing voltage characteristic, such must be. takeni'nto consideration. Therefore, oscillations can be effected relatively easily and the control must be very effective.

This means that if the effective heating current is changed, the electronic switch in the heating circuit should be activated as quickly as possible in such a way that the aforementioned change of the heating current is counteracted. The control, itself, takes place by way of an auxiliary direct voltage G. This approach is taken in an arrangement according to the invention, because this direct voltage can be considered a valuation of the heating power which is introduced into the silicon rod. It is therefore achieved that this direct voltage G is in proportion to the heating current. It is important to provide that the voltage G follows the changes of the heating current as quickly as possible. On the other hand, the direct voltage G should be free. of pulsation, since otherwise a difference formation with the desired reference value G would lead to a fluctuating differential voltage A which controls a pulse generator for activating an electronic switch in the heating circuit. In such a case, also an accurate input of the pulses controlling the electronic switch in the heating circuit cannot be achieved.

The direct voltage G is directly derived from the heating circuit. The transit time should be as small as possible, which means that the transmission should be as large as possible. It can be seen from the device illustrated in FIG. 1, which will be described in detail below, that a rectifying circuit II as well as the following circuits III and IV cause little delay, but that a delay counteracting the requirement for instant call of the automatic control system with respect to a change in the heating current is transmitted exclusively by the smoothing filter circuit 7.

For this reason, and in the interest of avoiding a noticeable pulsation of the voltage G, it is advisable to employ a steepened filter network, which will be described in detail with respect to FIG. 2, instead of one or several RC sections as smoothing four-pole networks.

Also the pulse generator IV, comprising the components l0 and 11, should react as soon as possible to changes of the voltage G. However, the time constant of such generators is usually negligible compared to delays provided by the smoothing filter circuit which is necessarily constructed of tandem-connected fourpole networks. Given the above mentioned requirements, a fast control of the heating current is achieved so that an interaction of the time constant of the automatic control system and the decreasing voltage current characteristic of the silicon rod, and therewith a slipping into an unstable condition, is avoided. 1

However, the arrangement which will be described below, results in the condition that the direct voltage G becomes proportional to the medium value of the current over half of the period of the voltage source 2, however, and not the effective value of the heating current. As can be seen from the respective definitions for the mean value, and

' I t 1! T L for the effective value, and the resulting ratio 1,, increases if I increases and therefore in the case of increased heating power. Therefore, a control according to I is successful. The fact that the voltage of the device which is described with respect to FIG. 2 becomes proportional to 1,, rather than I m results due to the Fourier description of the rectified voltage.

Therefore, according to the invention, it is provided in the above described device that the heating current I (t) operates by way of a second circuit, a rectifier circuit, which is inductively coupled to the heating circuit and to a smoothing four-pole filter circuit. The difference A G G of the direct current G which is received at the output of the filter circuit with respect to its desired value G controls a generator for the auxiliary voltage V (t) in such a way that the larger the difference the earlier the auxiliary voltage V (t) is applied.

BRIEF DESCRIPTION OF THE DRAWINGS Other objects, features and advantages of the invention, its organization, construction and operation, will be best understood from the following detailed description of preferred embodiments thereof taken in conjunction with the accompanying drawings, on which:

FIG. 1 is a schematic circuit diagram of a device for electrically heating a growing semiconductor rod;

FIG. 2 is a schematic diagram of a four-pole filter circuit which may be employed in the circuit of FIG. 1;

FIGS. 3 and 4 are schematic diagrams of apparatus for providing the auxiliary voltage employed in the circuit of FIG. 1;

FIG. 5 is a schematic diagram of the application of circuits, according to the present invention, in connection with the three phase supply; and

FIG. 6 is a schematic diagram of another embodiment of a pulse generator for providing the auxiliary voltage in accordance with the teachings of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The device schematically illustrated in FIG. 1 comprises a heating circuit I, circuits II and III serving for the creation of the direct voltage difference A, and a circuit IV serving for the production of the auxiliary voltage V (t). g

The heating circuit I comprises an alternating voltage source 2, an electronic switch 3, the primary winding of a current transformer 4 and the semiconductor rod 1 which is to be heated, and the required connections therebetween. Preferably, the alternating voltage source 2 is formed by the secondary winding of a transformer which is connected to the electrical mains, (f= 50 Hz); it supplies the heating current for the silicon rod 1 directly.

In addition to the feed lines supplying the heating current, the electronic switch 3 has acontrol electrode, a blocking or ignition electrode, and is realized, for example, by a thyratron tube or by an electronic semiconductor switch, particularly a thyristor, triac or switching transistor. The electronic switch 3 is designed in such a way that the heating current is blocked in the absence of the control voltage V (t). If the electronic switch 3 is activated, it blocks the heating current supply by the current source automatically in the following zero crossing of the operating voltage. Therefore, it must be ignited again in order to allow current passage. This task is fulfilled by application of the auxiliary voltage V (t). Preferably, the connection between the heating circuit I and the rectifier circuit II which has the purpose of deriving the direct voltage G, is formed by a current transformer 4, whose primary winding is located in the heating circuit I, whereas, the secondary side comprises many turns and forms a part of the rectifier circuit II.

By means of the rectifier 5 and the resistor 6, a pulsating direct current is produced from the alternating current which is induced from the transformer 4 onits.

secondary side, whereby this pulsating direct current is smoothed by means of a four-pole circuit. The direct voltage G occurs at the output of the four-pole circuit.

. and G are applied to the input of a direct current amplifier 10 in such a way that the amplifier 10 is operated by the difference A G G, or a direct voltage (I) which is proportional the voltage A, respectively. A corresponding voltage occurs at the output of the direct current amplifier 10 and controls the generator 11 for producing the control voltage V (t) which is applied to the electronic switch 3 of the heating circuit I. The elements 10 and 11 form the circuit IV.

The connections between the circuits I and II may also be provided by other known coupling elements, e.g. a low resistance shunt, instead of using the current transformer 4.

As was previously pointed out, the rectifier circuit II fulfills the task of deriving a direct voltage G which should be proportional to the effective value of the heating current, and which follows as quickly as possible changes in the effective value of the heating current. For this purpose, the voltage G is determined by the actual heating current. It would be best if the voltage G became proportional to the effective value of the actual power of the heating current in the heating circuit I. However, in many cases it is sufficient if the voltage G is made proportional to the current transmitted during a half cycle of the heating alternating current. This aim can be achieved by means of the switching means illustrated in the circuit II.

In the easiest case, the rectifier 5 can be a-half-wave rectifier. It is preferred, however, that a four-wave rectifier be utilized, which in the sample embodiment is constituted by a rectifier bridge. The rectifier is connected to and operates into a resistor 6, e.g. ohm,

whereupon a pulsating direct voltage will occur. The full wave rectifier 5 should always be used if the electronic switch 3 is to be opened in the heating circuit I in the case of positive as well as negative polarity of the current. The pulsating direct voltage occurring at the resistor 6 is smoothed by the fourpole circuit 7. For this purpose, it is preferable that the alternating voltage parts of the pulsating direct voltage occurring at the re 1 sistor 6 be suppressed. Accordingly, the four-pole circuit 7 to be used is designed as a basic low pass network, whereby the individual four-pole circuit consisting of series inductors and shunt capacitors can be conceived as T- or Trsections. At least part of the series inductors are bridged by capacitors, i.e., are extended to Zobel-sections, in order to achieve a greater steepness for damping in the frequency area of the altemating voltage parts of the rectified voltage. Furthermore, it is recommended to provide the four-pole circuit with an input and an output section according to Zobel. The easiest solution for the four-pole circuit would be an iterative network of T- and v-r-sections which are constructed of series resistors and shunt capacitors. However, such a network has a very low transmission capacity so that the voltage G provided at its output can only slowly follow the changes in the heating current. In addition, the remaining pulsation factor (ripple) would be very large if the RC components were used on an economically justifiable level. Therefore, it is more advisable if the four-pole network is constructed as a steepened filtering network, whereby the four-pole circuits should be as dissipation free as possible, by connecting a number of low pass filters having the same wave impedance behind each other in a network. By means of this four-pole circuit, the fundamental and the first harmonics of the operating voltage U (r) should be suppressed. This concept is based on the following thought: The pulsating direct voltage occurring at the resistor 6 can be illustrated as a straight function by a Fourier cosine line with a constant initial component. In other words, one deals with a direct current component. This direct current component is permitted to pass through the low pass filter network. Suitable low pass filters are provided for the fundamental and the first two or three harmonics of the alternating current flowing in the heating circuit I which filter the respective waves, but do not allow the direct current component to pass. In the case of the filtering network which will be described in more detail below, the input direct voltage reacts to changes of the heating current I (t) which have taken place previously over a maximum of two or three periods. This means that a practically instantaneously effective control is achieved.

A sample embodiment, corresponding to the principle of the foregoing discussion of filter arrangements, is illustrated in FIG. 2. The precondition which is applicable in most cases is chosen on a basis, particularly, that the voltage of the alternating current source 2 is sinusoidal. The input resistance of the resistor 6 of the rectifier circuit in the electrical circuit II, for example, amounts to 10 ohm, whereas, the wave impedance Z of the four-pole network is, e.g., adjusted to 1 ohm. For adjustment purposes, the input section of the four-pole network is provided with a component, according to Zobel, having an ohmic initial resistance Re of 0.8 Z (Z wave impedance of the components of the four-pole network), with a following series inductor L. and a parallel connected capacitance C,,, as well as a shunt capacitor C The input circuit is adjusted to a limit frequency which'slides below the net voltage, belowwhich the frequencies, which means the direct voltage according to the Fourier development, is allowed to pass and has its attenuation (damping) peak at the net frequency. Therefore, this section serves for the adjustment and the purpose of steepening the blockage damping at the net frequency. The next successive four-pole circuit of the network is a low pass filter and comprises a series inductor L and a shunt capacitor C It serves the damping of higher frequencies. The third four-pole circuit is constructed of a series inductor L a parallel shunt capacitor C bridging the same and a shunt capacitor C in such a way that, for example, the first harmonic, e.g. Hz is suppressed. The following section of the four-pole network comprises a series inductor L a capacitor C, which is connected parallel with the inductor L and a shunt capacitor C and is designed in such a way that the next harmonic, e.g. Hz, is blocked. The last section, which is formed by the parallel connection of an inductor L and a capacitor C connected in parallel therewith has a terminal shunt impedance of R which equals 0.8 Z, and the section corresponds in dimension to the input section according to the principles of Zobel. The entire arrangement is constructed as a low pass filter network with distinct poles in such a way that currents with-a frequency of lower than 40 Hz, that means direct current-according to Fourier, passes the network with practically no attenuation, whereas, the net frequency and all. higher frequencies, in particular the first harmonics, are filtered out.

It should be pointed out that the filter network 7 may also be designed in any way. The fundamental, for example, can also be filtered out-and smoothed at the output by way of the correspondingly dimensioned RC section. The steepness of the four-pole network is limited because of its transmission speed, as can be seen from the above example. In general, it is sufficient only to provide poles for the first three or four harmonics and the fundamental. In the sample embodiment according to FIG. 2, the four-pole circuit is designed in such a way that only the present direct current component is available. This direct current component, as can be easily recognized, proportional to the means value of the alternating heating current, measured over half of a period of the voltage of the alternating current source 2 in the heating network I.

The direct voltage G will be received at the output of the fourpole network 7, as was previously mentioned. This voltage is applied with the simultaneously created direct voltage G to the input of the differential amplifier, or to the control amplifier 10. However, the difference of the two direct voltages G and G can also be created by way of a bridge and then applied to a direct voltage amplifier 10. Finally, there is a possibility to utilize the direct voltage A without amplification for creating the pulses at the output of the generator 11. The differential or control amplifier 10 is a direct voltage amplifier of common construction, such as is utilized for example in control systems. The voltage d 04A appears at the output, whereby a is generally larger than 1. The pulse generator 11 is operated or controlled by either the voltage A or the amplified voltage qb. The emitted voltage pulses V (t) is applied between the control electrode of the electronic switch 3 and its input or output terminal, respectively. Steps have to be taken in order to prevent the electrical circuit holding voltage V (r) from producing a short circuit in a blocked condition of the switch 3, or to avoid its transit into the blocking condition during zero passage of the heating voltage U (r), respectively.

A simple arrangement is illustrated in FIGS. 3 and 4 for providing the voltage V (t) in response to the application of the voltage A or the amplified voltage d). The direct voltage A or the voltage d), respectively, charges the capacitor 22 by way of an impedance 21. If the voltage of this capacitor exceeds a certain value, a discharge takes place by way of a glow discharge path including the glow discharge device 23. The primary winding of a transformer 24 is connected, with two secondary windings, in the discharge circuit. The secondary winding 24a operates into the control circuit of the switch 23, here illustrated as a pnpn diode. The electronic switch 3 is actuated as soon as the capactor 22 is discharged. The other secondary winding 24b is connected to the control electrode of the pntn diode switch S in such a way that the switch 5 closing the circuit is interrupted when the capacitor 22 is discharged.

The switch S employed for applying the voltage A or the voltage (1), respectively, to the capacitor 22 comprises a thyristor which is provided with two separate control electrodes, and is illustrated as a pnpn diode whose two middle zones are provided with respective control electrodes. The activation of the switch S takes place through the application of pulses, which occur in synchronism with the zero crossings of the alternating voltage U (t) in the heating network I. For this purpose, FIG. 4 illustrates that an alternating voltage source 2, which is in synchronism with the operating alternating voltage source 2 in the heating network I is provided, which source 2 produces a pulsating alternating voltage by way of a four-wave rectifier 26 and a resistor 27. This alternating voltage is applied to a differentiating circuit, e.g. a capacitor 28a and a resistor 28b and to a transformer 29. The secondary winding of the transformer 29 is connected to one of the two control electrodes of the switch S in such a way that the voltage pulses render the switch permeable for the charging current. The rectified voltage at the resistor 27 has a point of discontinuity of its first shunt conductance, according to the time t during periods which coincide with the zero passage of the voltage U (r) in the heating network I. These points of discontinuity are the reason for the occurrence of steep voltage pulses at the secondary side of the transformer 29. These pulses are transmitted to the second control electrode of the switch 5 and are poled in such a way that they block the switch S for the charging current. Consequently, the switch S is activated for the charging current with every zero crossing of the heating voltage U (t) and turned off with every discharge of the capacitor 22.

The device illustrated in FIG. 1 is first of all intended for operation with single phase alternating current. However, the principle in this arrangement allows the application of the circuit to multi-phase supplies. This is illustrated in FIG. 5. A three-phase alternating voltage is made available by way of a three-phase transformer 30. At the secondary side of the transformer 30 each phase is provided with an electrical current. These are designated corresponding to the phases V V,,, V,.

Each of these electrical circuits is provided with an electronic switch 30, 3b, 30 corresponding to the electronic switch 3 of the arrangement according to FIG. 1. The control electrodes of the electronic switches are operated by individual voltages V (t), V,, (t), and V, (t). These voltages are derived by way of individual electrical circuits IIa, IIIa, IVa, IIb, IIIb, IVb, IIc, IIIc, IVc which are constructed according to the specification and to FIGS. 1-4. For this purpose, the currents pulsating in the circuits V V V, are scanned, which may be carried out by corresponding current transformers which are not illustrated in the drawing. The circuits V V,,, V,. are terminated by the primary windings of three respective transformers 31a, 31b, 310. The voltages occurring on the secondary side of these transformers are rectified and added by way of four-way rectifiers 32a, 32b, and serve for applying heating current to the silicon rod 1.

FIG. 6 illustrates a further sample embodiment of the pulse generator 11. An alternating voltage which is synchronous with the heating voltage is transformed by the transformer 61 to a suitable value and subsequently rectified in the rectifier bridge circuit 62. Therefore, a 100 Hz half-wave'voltage is available as a supply volt age. This voltage is applied to an RC circuit which is formed by a capacitor 63, the stabilization and limiting resistor 64 and a transistor 65. Depending on the magnitude of the control voltage supplied to the base of the transistor 65, whereby this control voltage may be the voltage A or the voltage (b, the control condition of the transistor 65 and therewith the time constant of the RC circuit change. The switch through voltage of the subsequently switched trigger element 67, e.g. the glow discharge path transistor trigger etc can be achieved at a point within a half wave, which is proportional to the height of the control voltage. The auxiliary thyristor 67, which is connected in series with the transformer 68 across the supply voltage via the rectifier 62 is simultaneously activated whereby the supply voltage V (t) is switched via the transformer 68 to the control electrode of the electronic switch 3 in the heating network I. The auxiliary thyristor 67 is blocked at the end of each half wave so that with each half wave of the alternating heating voltage the electronic switch 3 is rendered conductive at the moment which is required for the stabilization of the heating current network.

If the heating arrangement comprises a multi-phase alternating voltage source, the arrangement can be switched in each phase of the multiphase alternating voltage. However, it is also sufficient to switch an arrangement in one phase, whereby the ignition control pulses are staggered as required for the individual phases and are derived by way of correspondingly arranged amplifiers.

Although I have described my invention by reference to specific illustrative embodiments thereof, many changes and modifications of my invention may become apparent to those skilled in the art without departing from the spirit and scope of the invention. I therefore intend to include within the patent warranted hereon all such changes and modifications as may reasonably and properly be included within the scope of my contribution to the art.

What I claim is:

1. Apparatus for electrically heating a growing semiconductor rod during a deposition process, comprising: a heating current source including a source of altemating voltage connected to the semiconductor rod; an electronic switch connected in series with the rod and said source of alternating voltage to form a heating current circuit; a source of auxiliary voltage having an output voltage which varies with time connected to control said electronic switch to control the flow of heating current through the rod; a current transformer; a second current circuit coupled to said heating current circuit by said current transformer for providing a rectified signal representative of the heating current; a smoothing filter network connected to said second current circuit to provide a d.c. voltage from said rectified signal; and means connected between said filter network and said source of auxiliary voltage for providing a difference voltage between said d.c. voltage and a d.c. reference voltage, said source of auxiliary voltage operable to render said electronic switch conductive more quickly responsive to the larger the value of said difference voltage.

2. The apparatus of claim 1, wherein said electronic switch comprises a thyristor.

3. The apparatus of claim 1, wherein said electronic switch comprises a triac.

4. The apparatus of claim 1, wherein said second current circuit comprises a full-wave rectifier having an input connected to said current transfonner and an output, and a low value resistor connected across the output of said full-wave rectifier.

5. Apparatus for electrically heating a growing semiconductor rod during a deposition process, comprising: a heating current source including a source of alternating voltage connected to the semiconductor rod; an electronic switch connected in circuit with the rod and said source of alternating voltage to form a heating current circuit; a source of auxiliary voltage having an output voltage which varies with time connected to control said electronic switch to control the flow of heating current through the rod; a second current circuit coupled to said heating current circuit for providing a pulse rectified signal representative of the heating current; a smoothing filter network connected to said second current circuit to provide a d.c. voltage from said rectified signal, said smoothing filter network including a fourpole low pass network which includes means for filtering out the fundamental and at least the first two harmonics of the frequency of the heating circuit alternating voltage; and means connected between said filter network and said source of auxiliary voltage for providing a difference voltage between said d.c. voltage and a d.c. reference voltage, said source of auxiliary voltage operable to render said electronic switch conductive more quickly responsive to the larger the value of said difference voltage.

6. The apparatus of claim 5, wherein said filter network comprises a Zobel half section for matching adjustment of the filter network to said second current circuit, and successive low pass filter sections whose respective resonant frequencies correspond to the frequencies to be filtered out.

7. The apparatus of claim 6-,.wherein said filter network further includes another Zobel half section as an output section connected in succession behind the last low pass filter section.

8. Apparatus for electrically heating a growing semiconductor rod during a deposition process, comprising: heating current source including a source of alternating voltage connected to the semiconductor rod; an electronic switch connected in circuit with the-rod and said source of alternating voltage to form a heating current circuit; a source of auxiliary voltage having an output voltage which varies with time connected to control said electronic switch to control the flow of heating current through the rod; a second current circuit coupled to said heating current circuit for providing a rectified signal representative of the heating current; a smoothing filter network connected to said second current circuit to provide a d.c. voltage from said rectified signal; and means connected between said filter network and said source of auxiliary voltage for providing a difference voltage between said d.c. voltage and a d.c reference voltage including a direct current amplifier for amplifying said difference voltage to operate said source of auxiliary voltage, said source of auxiliary voltage operable to render said electronic switch conductive more quickly responsive to the larger the value of said difference voltage, said source of auxiliary voltage comprising a capacitor which is charged in response to the amplified difference voltage and discharge means for said capacitor operable in response to a predetermined voltage across said capacitor, said capacitor providing voltage pulses in response to the charging and discharging thereof as the auxiliary voltage for controlling said electronic switch.

9. The apparatus of claim 8, wherein said source of auxiliary voltage comprises a switch connected between said amplifier and said capacitor and means for operating said switch in synchronism with the heating current alternating voltage so that said switch is closed to apply the difference voltage to said capacitor at zero crossings of the heating alternating wave, said switch connected to said discharge means and operated to open in response to discharging of said capacitor.

10. The apparatus of claim 9, wherein said switch includes a pnpn diode having two control electrodes respectively connected to said means for operating said switch and said discharge means. 7

a 11. The apparatus of claim 10, wherein said discharge means includes a further transformer having a primary winding connected in a discharge path with said capacitor, a first secondary winding connected to said electronic switch and a second secondary winding connected to the respective control electrode of said pnpn diode.

12. The apparatus according to claim 10, wherein said means for operating said switch comprises a second source of alternating voltage synchronized with said heating current alternating voltage source, a fullwave rectifier connected to said second source, an output resistor connected to said full-wave rectifier, an output transformer having a primary winding connected in parallel with said output resistor and a secondary winding connected to the respective control electrode of said pnpn diode, and a differentiator comprising a capacitor connected between said output resistor and said primary winding for developing control pulses for said pnpn diode.

13. Apparatus for electrically heating a growing semiconductor rod during adeposition process, comprising: a heating current source including a source of alternating voltage connected to the semiconductor rod; an electronic switch connected in circuit with the rod and said source of alternating voltage to form a heating current circuit; a source of auxiliary voltage having an output voltage which varies with time convoltage for providing a difference voltage between said dc. voltage and a dc. reference voltage including a direct current amplifier for amplifying said difference voltage to operate said source of auxiliary voltage, said source of auxiliary voltage operable to render said electronic switch conductive more quickly responsive to the larger the value of said difference voltage. 

1. Apparatus for electrically heating a growing semiconductor rod during a deposition process, comprising: a heating current source including a source of alternating voltage connected to the semiconductor rod; an electronic switch connected in series with the rod and said source of alternating voltage to form a heating current circuit; a source of auxiliary voltage having an output voltage which varies with time connected to control said electronic switch to control the flow of heating current through the rod; a current transformer; a second current circuit coupled to said heating current circuit by said current transformer for providing a rectified signal representative of the heating current; a smoothing filter network connected to said second current circuit to provide a d.c. voltage from said rectified signal; and means connected between said filter network and said source of auxiliary voltage for providing a difference voltage between said d.c. voltage and a d.c. reference voltage, said source of auxiliary voltage operable to render said electronic switch conductive more quickly responsive to the larger the value of said difference voltage.
 2. The apparatus of claim 1, wherein said electronic switch comprises a thyristor.
 3. The apparatus of claim 1, wherein said electronic switch comprises a triac.
 4. The apparatus of claim 1, wherein said second current circuit comprises a full-wave rectifier having an input connected to said current transformer and an output, and a low value resistor connected across the output of said full-wave rectifier.
 5. Apparatus for electrically heating a growing semiconductor rod during a deposition process, comprising: a heating current source including a source of alternating voltage connected to the semiconductor rod; an electronic switch connected in circuit with the rod and said souRce of alternating voltage to form a heating current circuit; a source of auxiliary voltage having an output voltage which varies with time connected to control said electronic switch to control the flow of heating current through the rod; a second current circuit coupled to said heating current circuit for providing a pulse rectified signal representative of the heating current; a smoothing filter network connected to said second current circuit to provide a d.c. voltage from said rectified signal, said smoothing filter network including a four-pole low pass network which includes means for filtering out the fundamental and at least the first two harmonics of the frequency of the heating circuit alternating voltage; and means connected between said filter network and said source of auxiliary voltage for providing a difference voltage between said d.c. voltage and a d.c. reference voltage, said source of auxiliary voltage operable to render said electronic switch conductive more quickly responsive to the larger the value of said difference voltage.
 6. The apparatus of claim 5, wherein said filter network comprises a Zobel half section for matching adjustment of the filter network to said second current circuit, and successive low pass filter sections whose respective resonant frequencies correspond to the frequencies to be filtered out.
 7. The apparatus of claim 6, wherein said filter network further includes another Zobel half section as an output section connected in succession behind the last low pass filter section.
 8. Apparatus for electrically heating a growing semiconductor rod during a deposition process, comprising: heating current source including a source of alternating voltage connected to the semiconductor rod; an electronic switch connected in circuit with the rod and said source of alternating voltage to form a heating current circuit; a source of auxiliary voltage having an output voltage which varies with time connected to control said electronic switch to control the flow of heating current through the rod; a second current circuit coupled to said heating current circuit for providing a rectified signal representative of the heating current; a smoothing filter network connected to said second current circuit to provide a d.c. voltage from said rectified signal; and means connected between said filter network and said source of auxiliary voltage for providing a difference voltage between said d.c. voltage and a d.c. reference voltage including a direct current amplifier for amplifying said difference voltage to operate said source of auxiliary voltage, said source of auxiliary voltage operable to render said electronic switch conductive more quickly responsive to the larger the value of said difference voltage, said source of auxiliary voltage comprising a capacitor which is charged in response to the amplified difference voltage and discharge means for said capacitor operable in response to a predetermined voltage across said capacitor, said capacitor providing voltage pulses in response to the charging and discharging thereof as the auxiliary voltage for controlling said electronic switch.
 9. The apparatus of claim 8, wherein said source of auxiliary voltage comprises a switch connected between said amplifier and said capacitor and means for operating said switch in synchronism with the heating current alternating voltage so that said switch is closed to apply the difference voltage to said capacitor at zero crossings of the heating alternating wave, said switch connected to said discharge means and operated to open in response to discharging of said capacitor.
 10. The apparatus of claim 9, wherein said switch includes a pnpn diode having two control electrodes respectively connected to said means for operating said switch and said discharge means.
 11. The apparatus of claim 10, wherein said discharge means includes a further transformer having a primary winding connected in a discharge path with said capacitor, a first secondary winding coNnected to said electronic switch and a second secondary winding connected to the respective control electrode of said pnpn diode.
 12. The apparatus according to claim 10, wherein said means for operating said switch comprises a second source of alternating voltage synchronized with said heating current alternating voltage source, a full-wave rectifier connected to said second source, an output resistor connected to said full-wave rectifier, an output transformer having a primary winding connected in parallel with said output resistor and a secondary winding connected to the respective control electrode of said pnpn diode, and a differentiator comprising a capacitor connected between said output resistor and said primary winding for developing control pulses for said pnpn diode.
 13. Apparatus for electrically heating a growing semiconductor rod during a deposition process, comprising: a heating current source including a source of alternating voltage connected to the semiconductor rod; an electronic switch connected in circuit with the rod and said source of alternating voltage to form a heating current circuit; a source of auxiliary voltage having an output voltage which varies with time connected to control said electronic switch to control the flow of heating current through the rod; a second current circuit coupled to said heating current circuit for providing a rectified signal representative of the heating current; a smoothing filter network connected to said second current circuit to provide a d.c. voltage from said rectified signal; and means connected between said filter network and said source of auxiliary voltage for providing a difference voltage between said d.c. voltage and a d.c. reference voltage including a direct current amplifier for amplifying said difference voltage to operate said source of auxiliary voltage, said source of auxiliary voltage operable to render said electronic switch conductive more quickly responsive to the larger the value of said difference voltage. 